Exhaust gas purifying catalyst

ABSTRACT

An exhaust gas purifying catalyst including a carrier having a plurality of cells as exhaust gas passages, an HC adsorbent layer formed on the carrier of each of the cells, and an upper catalyst layer disposed on an upstream side of each of the exhaust gas passages on the HC adsorbent layer and a lower catalyst layer disposed on a downstream side of each of the exhaust gas passages on the HC adsorbent layer. The upper catalyst layer includes more O2-storage material than the lower catalyst layer, and the lower catalyst layer includes a catalyst having a wider activation range than the upper catalyst layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exhaust gas purifyingcatalyst that purifies carbon monoxide (CO), hydrocarbons (HC) andnitrogen oxides (NOx) in exhaust gas discharged from an internalcombustion engine of a gasoline or diesel automobile, a boiler or thelike. More particularly, the present invention relates to an exhaust gaspurifying catalyst that purifies a large amount of HC discharged in alow temperature range at time of starting up an engine, in which athree-way catalyst is not activated.

[0003] 2. Description of the Related Art

[0004] Heretofore, in order to purify exhaust gas from an internalcombustion engine of an automobile or the like, a three-way catalystthat simultaneously performs oxidation for carbon monoxide (CO) andhydrocarbons (HC) and reduction for nitrogen oxides (NOx) has widelybeen used. However, since the temperature of the exhaust gas is low andthe three-way catalyst disposed on an exhaust gas passage does not reachan activation temperature immediately after the start-up of the engine,a large amount of cold HC discharged in this case cannot be purified.

[0005] Recent years, for the purpose of purifying such cold HC, anHC-adsorbing/purifying catalyst (HC-trap catalyst) as a three-waycatalyst having an HC adsorbing function has been developed, whichincludes a hydrocarbon adsorbent (HC adsorbent) and a purifying catalystsuch as a three-way catalyst.

[0006] The HC-trap catalyst temporarily adsorbs and holds cold HCdischarged in the low temperature range at the time of starting up theengine, in which the three-way catalyst is not activated. Then, theHC-trap catalyst gradually desorbs the HC and purifies the desorbed HCby the purifying catalyst when the three-way catalyst is activated dueto a temperature increase of the exhaust gas.

[0007] As the HC adsorbent, zeolite is generally used. As the catalystpurifying the HC desorbed from the HC adsorbent, a catalyst obtained bymixing noble metal species such as rhodium (Rh), platinum (Pt) andpalladium (Pd) on the same layer and a catalyst of a multilayerstructure including Rh and Pd layers have been proposed.

[0008] Japanese Patent Laid-Open Publication H2-56247 (published in1990) discloses an exhaust gas purifying catalyst including a firstlayer mainly containing zeolite and a second layer provided on the firstlayer. The second layer mainly contains noble metals such as Pt, Pd andRh.

[0009] Other HC-trap catalysts have been disclosed in Japanese PatentLaid-Open Publications H6-74019 (published in 1994), H7-144119(published in 1995), H6-142457 (published in 1994), H5-59942 (publishedin 1993), H7-102957 (published in 1995), H7-96183 (published in 1995)and H11-210451 (published in 1999).

SUMMARY OF THE INVENTION

[0010] In the case of using the conventional HC-trap catalyst, the coldHC adsorbed to the HC adsorbent at the time of starting up the enginestarts to be desorbed as the temperature increases. However, since thestarting temperature of HC desorption is lower than the startingtemperature of three-way catalyst activation, early-desorbed HC isdischarged without being purified by the three-way catalyst in theHC-trap catalyst. In order to further improve purification efficiencyfor HC in the exhaust gas purifying system as a whole, it is necessaryto control the discharge of such unpurified HC.

[0011] Moreover, since an oxidation reaction occurs when the adsorbed HCis desorbed and purified, an atmosphere around the purifying catalystlayer of the HC-trap catalyst falls in a state of oxygen shortage. Sincethe three-way catalyst exhibits the best action of the purifyingcatalyst in the range of the stoichiometric air/fuel ratio, it cannotexert the action of the purifying catalyst sufficiently in theatmosphere where oxygen is short. Therefore, HC, CO and NOx cannot bepurified in good balance, and it becomes difficult to enhance thepurification efficiency for the cold HC sufficiently.

[0012] In order to enhance the purification efficiency for the cold HC,the following purifying methods have been studied. In one purifyingmethod, a switching of an exhaust gas passage controls adsorbed HC to bedesorbed after the three-way catalyst is sufficiently activated, and thedesorbed HC is purified by the three-way catalyst. In another purifyingmethod, an electric heater raises the temperature of a three-waycatalyst to accelerate an activation of the three-way catalyst. In theother purifying method, an introduction of external air accelerates anactivation of the three-way catalyst. However, these methods are costlybecause of complex system configurations, and purification efficiencyfor the cold HC cannot be sufficiently enhanced.

[0013] Moreover, in an HC-trap catalyst, the temperature and gasatmosphere differ on the upstream side and the downstream side in theexhaust gas flow that is discharged from the engine into the HC-trapcatalyst. The temperature of the exhaust gas is high on the upstreamside closer to the engine, and is lowered toward the downstream. Such adifference between the upstream and the downstream in the temperaturecondition occurs also in a single HC-trap catalyst. For example, in anHC-trap catalyst that uses a honeycomb carrier having a plurality ofcells serving as exhaust gas passages, the temperature of the exhaustgas flowing in each cell differs on the upstream side and the downstreamside.

[0014] Moreover, since an adsorbing/desorbing reaction for HC and apurifying reaction for HC occur in each cell, the gas atmosphere alsodiffers in the upstream region and the downstream region as thesereactions proceed. However, since the same configuration and compositionare formed from the upstream region to the downstream region in theHC-trap catalyst of the conventional type, the optimum configuration andcomposition have not been realized in each region of the HC-trapcatalyst.

[0015] An object of the present invention is to provide an exhaust gaspurifying catalyst that purifies HC discharged in the low temperaturerange at the time of starting up the engine more efficiently.

[0016] An exhaust gas purifying catalyst according to a first aspect ofthe present invention includes a carrier having a plurality of cells asexhaust gas passages, an HC adsorbent layer formed on the carrier ofeach of the cells, an upper catalyst layer disposed on an upstream sideof each of the exhaust gas passages on the HC adsorbent layer and alower catalyst layer disposed on a downstream side of each of theexhaust gas passages on the HC adsorbent layer. The upper catalyst layercontains more O₂-storage material than the lower catalyst layer. Thelower catalyst layer contains a catalyst having a wider activation rangethan that of the upper catalyst layer. Note that the catalyst having thewider activation range means a catalyst having wide temperature and gasatmosphere conditions required for exerting a catalytic function.

[0017] An exhaust gas purifying catalyst according to a second aspect ofthe present invention includes a carrier having a plurality of cells asexhaust gas passages, an HC adsorbent layer formed on at least anupstream region on the carrier of each of the cells, and a purifyingcatalyst layer formed on the HC adsorbent layer. Here, a substantialcross-sectional area on a downstream side of the exhaust gas passage ismade narrower than a substantial cross-sectional area on an upstreamside thereof. Moreover, the purifying catalyst layer includes an uppercatalyst layer disposed on an upstream side of each of the exhaust gaspassages and a lower catalyst layer disposed on a downstream side ofeach of the exhaust gas passage. The upper catalyst layer contains moreO₂-storage material than the lower catalyst layer. The lower catalystlayer contains a catalyst having a wider activation range than that ofthe upper catalyst layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a perspective view of an exhaust gas purifying catalystaccording to a first aspect of the present invention.

[0019]FIGS. 2A and 2B are enlarged cross-sectional views, each showing across section perpendicular to an exhaust gas flow in a cell of theexhaust gas purifying catalyst of the first embodiment, and FIG. 2C isan enlarged cross-sectional view showing a cross-section parallel to theexhaust gas flow in the cell.

[0020]FIG. 3 is an enlarged cross-sectional view showing a cross sectionparallel to an exhaust gas flow in a cell of another exhaust gaspurifying catalyst in the first embodiment.

[0021]FIG. 4 is an enlarged cross-sectional view showing a cross sectionparallel to an exhaust gas flow in a cell of an exhaust gas purifyingcatalyst of a comparative example according to the first embodiment.

[0022]FIG. 5 is a view showing a configuration of a purifying system foruse in evaluating purification efficiencies of the catalysts.

[0023]FIGS. 6A and 6B are tables showing conditions of exhaust gaspurifying catalysts in examples I-1 to I-11 and comparative examples I-1to I-5 according to the first embodiment, and FIG. 6C is a table showingtotal amounts of noble metals contained in the exhaust gas purifyingcatalysts, and showing HC adsorption rates and HC purification rates ofthe exhaust gas purifying catalysts, which have been measured by use ofthe purifying system shown in FIG. 5.

[0024]FIGS. 7A and 7B are enlarged cross-sectional views, each showing across section perpendicular to an exhaust gas flow in a cell of anexhaust gas purifying catalyst of an example II-1 and II-10 of a secondembodiment, and FIG. 7C is an enlarged cross-sectional view showing across-section parallel to the exhaust gas flow in the cell.

[0025] FIGS. 8 to 11 are enlarged cross-sectional views respectivelyshowing cross sections parallel to exhaust gas flows in cells of exhaustgas purifying catalysts of examples II-2 to II-5 in the secondembodiment.

[0026] FIGS. 12 to 15 are enlarged cross-sectional views showing crosssections parallel to exhaust gas flows in cells of exhaust gas purifyingcatalysts of examples II-6 to II-9 and II-11 in the second embodiment.

[0027] FIGS. 16 to 21 are enlarged cross-sectional views respectivelyshowing cross sections parallel to exhaust gas flows in cells of exhaustgas purifying catalysts of comparative examples II-1 to II-5 related tothe second embodiment.

[0028]FIGS. 22A to 22C are tables showing conditions of the exhaust gaspurifying catalysts of the examples II-1 to II-11 and the comparativeexamples II-1 to II-6 according to the second embodiment, and FIG. 22Dis a table showing total amounts of noble metals contained in theexhaust gas purifying catalysts, and showing HC adsorption rates and HCpurification rates of the exhaust gas purifying catalysts, which havebeen measured by use of the purifying system shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] (First Embodiment)

[0030] As shown in FIG. 1, an exhaust gas purifying catalyst accordingto a first embodiment of the present invention is an HC-trap catalyst100 that has an HC adsorbent layer and a catalyst layer in a honeycombcarrier 10 having a plurality of cells 50 serving as exhaust gaspassages.

[0031] The HC-trap catalyst 100 according to the first embodiment ischaracterized by including a purifying catalyst layer composeddifferently on an upstream side and a downstream side of an exhaust gasflow.

[0032]FIGS. 2A to 2C show the cell structure of the HC-trap catalyst 100according to the first embodiment. As shown in FIGS. 2A to 2C, an HCadsorbent layer 20 mainly containing zeolite is formed on the carrier 10of each cell, and the purifying catalyst layer is formed on the HCadsorbent layer 20. The purifying catalyst layer is divided into anupper catalyst layer 311 disposed on the upstream side of the exhaustgas flow and a lower catalyst layer 321 disposed on the downstream sidethereof. The upper catalyst layer 311 contains more O₂-storage materialthan the lower catalyst layer 321. The lower catalyst layer 321 containsa catalyst having a wider activation range than that of the uppercatalyst layer 311.

[0033] The O₂-storage material is a material having an oxygen storagecapability. When a circumambient atmosphere thereof becomes short ofoxygen, the O₂-storage material exerts an oxygen emission function. Thecatalyst having the wide activation range means a catalyst having a widecondition showing an activation state, that is, the purifying catalystfunctions as a purifying under wide temperature and gas atmosphereconditions. Concretely, such a catalyst means a catalyst showing theactivation state under the condition where the temperature is lower orwhere the concentration of oxygen is lower.

[0034] The exhaust gas in the upstream region, which flows in the cell50, has high temperature because it is closer to the engine. The uppercatalyst layer 311 in contact with the exhaust gas raises itstemperature rapidly and is activated early, and the HC purifyingreaction proceeds. However, since oxygen is consumed in the HC purifyingreaction, the downstream region of the cell is apt to fall in theshortage of oxygen. When shortage of oxygen occurs, the activationfunction of the catalyst contained in the catalyst layer is lowered.However, in the HC-trap catalyst according to the first embodiment, theO₂-storage material contained much in the upper catalyst layer 311 emitsoxygen when the concentration of circumambient oxygen is lowered, andsolves the shortage of oxygen which is apt to occur on the downstreamside of the cell, in order to maintain the concentration ofcircumambient oxygen constantly. Thus, it is made possible to acceleratethe purifying reaction for HC desorbed from the HC adsorbent layer 20.

[0035] In the case of using an exhaust gas purifying system as shown inFIG. 5, since the three-way catalyst is provided upstream of the HC-trapcatalyst, oxygen in the exhaust gas is consumed by the catalyticreaction of the three-way catalyst after the three-way catalyst isactivated. Accordingly, the oxygen concentration is low even if theair/fuel ratio in the exhaust gas is stoichiometric ratio. When thepurifying reaction proceeds in the HC-trap catalyst, the oxygenconcentration of the downstream region of the cell becomes lower.Therefore, the oxygen emission effect in the upstream region of theHC-trap catalyst, which is caused by the O₂-storage material, becomesfar more important.

[0036] As the O₂-storage material, for example, a cerium oxide isavailable. Specifically, as such a cerium oxide, an oxide compound ofcerium and an element A, which is represented as Ce-[A]-Ob, isavailable. The element A is at least one selected from the groupconsisting of zirconium, lanthanum, yttria, praseodymium and neodymium.

[0037] Each of the upper catalyst layer 311 and the lower catalyst layer321 contains a noble metal as a catalyst component supported on theoxide and the like. Since the cerium oxide as the O₂-storage materialhas a smaller BET surface area than alumina and the like, it cannotsupport the noble metal expressing the catalytic function in highdispersion. Accordingly, it is desirable to mainly use alumina that cansupport the noble metal in higher dispersion than the cerium oxide forthe lower catalyst layer 321. The noble metal as the catalyst issupported on the alumina in high dispersion, and thus the contactefficiency for the exhaust gas and the catalyst is enhanced, resultingin the widening of an activation range of the catalyst. As describedabove, since the temperature of the exhaust gas on the downstream sideof the cell 50 is lower than that on the upstream side, the rise oftemperature in the lower catalyst layer 321 is slower than in theupstream side. However, since the lower catalyst layer 321 contains thecatalyst having the wider activation range than the upper catalyst layer311, the lower catalyst layer 321 can start the activation even in theregion where the concentration of oxygen is lower. Therefore, thepurifying reaction for the desorbed HC can be accelerated.

[0038] It is desirable to mainly use Ce-[A]-Ob as a catalyst supporterin the upper catalyst layer 311 and to mainly use alumina as a catalystsupporter in the lower catalyst layer 321.

[0039] Moreover, it is desirable to start the activation of the uppercatalyst layer 311 earlier and to use a catalyst activated in a rangefrom 150 to 300° C. Accordingly, it is desirable to use Pd in which thestarting temperature of activation is relatively low as a catalystcomponent for the upper catalyst layer 311. Meanwhile, for the lowercatalyst layer 321, it is desirable to use a catalyst that is activatedin the relatively low temperature and has a wide activation rangecapable of being activated even in the state of oxygen shortage.Therefore, it is desirable to use a catalyst containing Pt, Rh or thelike in addition to the catalyst containing Pd.

[0040] Concretely, in the cell shown in FIG. 2C, the upper catalystlayer 311 mainly uses Pd-supported Ce-[A]-Ob, and the lower catalystlayer 321 mainly uses Pd supported Ce—Al₂O₃. Note that Ce—Al₂O₃represents Ce doped Al₂O₃. Since the Ce—Al₂O₃ contains Ce, oxygenemission action is obtained. Also the Ce—Al₂O₃ can support Pd in highdispersion since alumina is a base material. Therefore, the contactefficiency and contact time of the exhaust gas and Pd as the catalystare enhanced, thus making it possible to activate the catalyst at thelower temperature and the lower concentration of oxygen. Specifically,since the lower catalyst layer 321 has the wide activation range inaccordance with the catalyst component on the upstream side, it ispossible to improve the HC purification efficiency of theHC-trap-catalyst 100 as a whole.

[0041] Note that Pd-supported Ce—Al₂O₃ may be added to the uppercatalyst layer 311. In this case, dispersibility of Ce and Pd isimproved, and the HC purification efficiency can be improved.

[0042] The lower catalyst layer 321 may contain any of Pd-supportedCe—Al₂O₃, Pt-supported Ce—Al₂O₃, Rh-supported Zr—Al₂O₃, Pt-supportedLaCe—ZrO₂, or an arbitrary combination thereof.

[0043] In the exhaust gas purifying catalyst according to the firstembodiment, preferably, the carrier is of a monolithic structure, andthe upper catalyst layer is provided in a range from 50 to 90% of theoverall length of the carrier from the upstream side to the downstreamside.

[0044] The honeycomb carrier can be separated into two carriers on theupstream side and the downstream side, and catalyst layers different incomposition from each other can be formed on the two separated carriers,respectively. However, when the monolithic structure is employed, andthe upper and lower catalyst layer 311 and 321 different in compositionfrom each other are provided in one carrier, then the escape of heat tothe outside can be rather controlled, and the lower catalyst layer 321can be rather heated up early to be activated. Therefore, in this case,it is possible to obtain higher HC purification efficiency than in thecase when the carrier is separated into two.

[0045] Although zeolite can be used as the HC adsorbent layer 20, noparticular limitations are imposed on materials therefor. In the case ofusing zeolite, adsorptivity thereof for the cold HC is affected by arelationship between the composition of HC species in the exhaust gasand a pore diameter of the zeolite. Therefore, it is preferable toselect and use zeolite having the optimum pore diameter, distributionand skeletal structure.

[0046] Although an MFI type is generally used, zeolite having anotherpore diameter, for example, USY is singly used, or plural types of suchzeolites are mixed, and thus the pore diameter distribution of zeoliteis controlled. However, after a long-time use, because of differences indistortion of pore diameter and adsorption/desorption characteristicsdepending on types of zeolites, adsorption of the HC species in theexhaust gas becomes insufficient.

[0047] As an HC adsorbent used for the HC adsorbent layer 20, H typeβ-zeolite having a Si/2Al ratio set at a range of 10 to 1000 isavailable. Since this H type β-zeolite has a wide pore distribution andhigh heat resistance, the H type β-zeolite is suitable from a viewpointof improvements of the HC adsorption efficiency and heat resistance.

[0048] In addition, if one selected from MFI, Y type zeolite, USY,mordenite and ferrierite or an arbitrary mixture thereof is used as theHC adsorbent in combination with the H type β-zeolite, then the porediameter distribution of the material can be expanded. Therefore, the HCadsorption efficiency of the HC adsorbent layer can be further improved.

[0049] For the HC adsorbent layer 20, besides the above zeolite-basedmaterials, one selected from palladium (Pd), magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba), silver (Ag), yttrium (Y), lanthanum(La), cerium (Ce), neodymium (Nd), phosphorus (P), boron (B) andzirconium (Zr) or a mixture thereof can be added. Since the adsorptivityand heat resistance of zeolite can be accordingly enhanced more, it ispossible to delay the desorption of the adsorbed HC.

[0050] In addition, the HC adsorbent layer 20 may contain theabove-described zeolite as a main component, and may further contain oneselected from Pt, Rh and Pd or a mixture thereof, zirconium oxidecontaining 1 to 40 mol %, in metal, of one selected from Ce, Nd,praseodymium (Pr) and La or a mixture thereof, and alumina. Accordingly,since the purifying catalyst components are added to the HC adsorbentlayer 20, it is possible to improve the purification efficiency for thedesorbed HC.

[0051] No particular limitations are imposed on materials for thehoneycomb carrier 10, and conventionally known materials can be used.Concretely, cordierite, metal and silicon carbide can be used.

[0052] Although the HC-trap catalyst according to the first embodimenthas been described above, the upper catalyst layer and the lowercatalyst layer do not necessarily have to be located in two completelyseparate regions, and may be partially overlapped each other. Inaddition, the HC-trap catalyst may be formed such that the compositionthereof is gradually changed from the upstream side to the downstreamside.

EXAMPLES I

[0053] The table of FIG. 6A shows specifications of catalysts inexamples I-1 to I-9, and the table of FIG. 6B shows specifications ofcatalysts in comparative examples I-1 to I-5.

Example I-1

[0054]FIGS. 2A to 2C show the structure of the HC-trap catalyst of theexample I-1. In the HC-trap catalyst of the example I-1, the uppercatalyst layer 311 contains Pd-supported Ce-[A]-Ob. Concretely, asCe-[A]-Ob, La_(0.01)Ce_(0.69)Zr_(0.3)Ob was used. The lower catalystlayer 321 contains Pd-supported Ce—Al₂O₃.

[0055] The respective catalyst layers were prepared by the followingmethods.

[0056] <HC Adsorbent Layer 20>

[0057] 800 g of β-zeolite powder (Si/2Al=35), 1333.3 g of silica sol(solid part 15%) and 1000 g of pure water were poured into a ball millpot made of alumina, then were milled for 60 minutes, and thus a slurrysolution was obtained. This slurry solution was coated on a monolithiccarrier of 300 cells/6 mills (46.5 cells/cm², wall thickness 0.0152 cm)and of a catalyst capacity 1.0 L, dried for 30 minutes in an air flow of50° C. after removing extra slurry in the cells by an air flow, thenbaked at 400° C. for an hour after drying for 15 minutes in an air flowof 150° C. The weight of the coating layer after the baking was 350g/dm³. Thus, the HC adsorbent layer 20 was obtained.

[0058] <Upper Catalyst Layer 311>

[0059] Cerium oxide powder (Ce 69 mol %) containing 1 mol % of La and 30mol % of Zr was impregnated with a palladium nitrate aqueous solution,or sprayed therewith while the cerium oxide powder was stirred at a highspeed. After the cerium oxide powder was dried at 150° C. for 24 hours,the dried cerium oxide powder was baked at 400° C. for an hour, and thenat 600° C. for an hour, and thus Pd-supported cerium oxide powder(powder-b) was obtained. The Pd concentration of this “powder-b” was1.0%.

[0060] 314 g of the foregoing Pd-supported cerium oxide powder(powder-b), 190 g of nitric acid alumina sol (19 g, in Al₂O₃, of solobtained by adding 10% of nitric acid to 10% of boehmite alumina) and2000 g of pure water were poured into a magnetic ball mill, then weremixed and milled, and thus a slurry solution was obtained. This slurrysolution was coated on one-third portion of the foregoing HC adsorbentlayer 20, which corresponds to the exhaust gas entrance side (upstreamside), dried after removing extra slurry in the cells by an air flow,and baked at 400° C. for an hour. The weight of the coating layer afterthe baking was 33.3 g/dm³. Thus, the upper catalyst layer 311 wasobtained.

[0061] <Lower Catalyst Layer 321>

[0062] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a palladium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pd-supported alumina powder (powder-a) was obtained. ThePd concentration of this “powder-a” was 4.0%.

[0063] 628 g of the foregoing Pd supported alumina powder (powder-a),140 g of nitric acid alumina sol (14 g, in Al₂O₃, of sol obtained byadding 10% of nitric acid to 10% of boehmite alumina), 25 g of bariumcarbonate (17 g of BaO) and 2000 g of pure water were poured into amagnetic ball mill, then were mixed and milled, and thus a slurrysolution was obtained. This slurry solution was coated on two-thirdsportion of the foregoing HC adsorbent layer 20, which corresponded tothe downstream side of the exhaust gas, dried after removing extraslurry in the cells by an air flow, and baked at 400° C. for an hour.The weight of the coating layer after the baking was 66.7 g/dm³. Thus,the lower catalyst layer 321 was obtained.

Example I-2

[0064]FIG. 3 shows the structure of the HC-trap catalyst of the exampleI-2. In the HC-trap catalyst of the example I-2, the upper catalystlayer 312 contains Pd-supported Ce-[A]-Ob and Pd-supported Ce—Al₂O₃.Similarly to the example I-1, as Ce-[A]-Ob, La_(0.01)Ce_(0.69)Zr_(0.3)Obwas used. The lower catalyst layer 322 contains Pt-supported Ce—Al₂O₃,Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂.

[0065] The HC adsorbent layer 20 was prepared in the similar method tothat of the example I-1, and the upper catalyst layer 312 and the lowercatalyst layer 322 were prepared by the following methods, respectively.

[0066] <Upper Catalyst Layer 312>

[0067] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a palladium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pd supported alumina powder (powder-c) was obtained. ThePd concentration of this “powder-c” was 8.0%.

[0068] Cerium oxide powder (Ce 69 mol %) containing 1 mol % of La and 30mol % of Zr was impregnated with a palladium nitrate aqueous solution,or sprayed therewith while the cerium oxide powder was stirred at a highspeed. After the cerium oxide powder was dried at 150° C. for 24 hours,the dried cerium oxide powder was baked at 400° C. for an hour, and thenat 600° C. for an hour, and thus Pd supported cerium oxide powder(powder-d) was obtained. The Pd concentration of this powder d was 4.0%.

[0069] 200 g of the foregoing Pd supported alumina powder (powder-c), 71g of the foregoing Pd supported cerium oxide powder (powder-d), 120 g ofnitric acid alumina sol (12 g, in Al₂O₃, of sol obtained by adding 10%of nitric acid to 10% of boehmite alumina), 50 g of barium carbonate (33g of BaO) and 1000 g of pure water were poured into a magnetic ballmill, then were mixed and milled, and thus a slurry solution wasobtained. This slurry solution was coated on one-third portion of the HCadsorbent layer 20 prepared in advance by the similar method to that ofthe example I-1, which corresponds to the exhaust gas entrance side(upstream side), dried after removing extra slurry in the cells by anair flow, and baked at 400° C. for an hour. The weight of the coatinglayer after the baking was 33.3 g/dm³. Thus, the upper catalyst layer312 was obtained.

[0070] <Lower Catalyst Layer 322>

[0071] Alumina powder (Al 97 mol %) containing 3 mol % of Zr wasimpregnated with a rhodium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Rh supported alumina powder (powder-e) was obtained. TheRh concentration of this “powder-e” was 1.5%.

[0072] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a dinitro diamine platinum aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pt supported alumina powder (powder-f) was obtained. ThePt concentration of this powder f was 1.5%.

[0073] Zirconium oxide powder containing 1 mol % of La and 20 mol % ofCe was impregnated with a dinitro diamine platinum aqueous solution, orsprayed therewith while the zirconium oxide powder was stirred at a highspeed. After the zirconium oxide powder was dried at 150° C. for 24hours, the dried zirconium oxide powder was baked at 400° C. for anhour, and then at 600° C. for an hour, and thus Pt supported zirconiumoxide powder (powder-g) was obtained. The Pt concentration of this“powder-g” was 1.5%.

[0074] 157 g of the foregoing Rh supported alumina powder (powder-e),236 g of the foregoing Pt supported alumina powder (powder-f), 236 g ofthe foregoing Pt supported zirconium oxide powder (powder-g) and 380 gof nitric acid alumina sol were poured into a magnetic ball mill, thenwere mixed and milled, and thus a slurry solution was obtained. Thisslurry solution was coated on two-thirds portion of the foregoing HCadsorbent layer 20 prepared in advance by the similar method to that ofthe example I-1, which corresponds to the exhaust gas emission side(downstream side), dried after removing extra slurry in the cells by anair flow, and baked at 400° C. for an hour. The weight of the coatinglayer after the baking was 66.7 g/dm³. Thus, the lower catalyst layer322 was obtained.

[0075] The total noble metal supported amounts of the upper catalystlayer 312 and the lower catalyst layer 322 were 0.71 g/dm³ for Pt, 1.88g/dm³ for Pd, and 0.24 g/dm³ for Rh.

Examples I-3 to I-6

[0076] The following HC-trap catalysts were prepared by use of thesimilar method to the method of the example I-1. As shown in FIGS. 2A to2C, each of the HC-trap catalysts contains Pd-supported Ce-[A]-Ob in theupper catalyst layer 311 and Pd-supported Ce—Al₂O₃ in the lower catalystlayer 321. As Ce-[A]-Ob, La_(0.01)Ce_(0.69)Pr_(0.3)Ob was used in theexample I-3, La_(0.01)Ce_(0.69)Nd_(0.3)Ob was used in the example I-4,La_(0.01)Ce_(0.69)Pr_(0.2)Nd_(0.1)Ob was used in the example I-5, andLa_(0.01)Ce_(0.69)ZrO₂Pr_(0.1)Ob was used in the example I-6.

Examples I-7 to I-11

[0077] The following HC-trap catalysts were prepared by use of thesimilar method to the method of the example I-2. As shown in FIG. 3,each of the HC-trap catalysts contains Pd-supported Ce-[A]-Ob andPd-supported Ce—Al₂O₃ in the upper catalyst layer 312, and containsPd-supported Ce—Al₂O₃, Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂ inthe lower catalyst layer 322 similarly to the example I-2.

[0078] As Ce-[A]-Ob, La_(0.01)Ce_(0.69)Pr_(0.3)Ob was used in theexample I-7, La_(0.01)Ce_(0.69)Nd_(0.3)Ob was used in the example I-8,La_(0.01)Ce_(0.69)Pr_(0.2)Nd_(0.1)Ob was used in the example I-9,La_(0.01)Ce_(0.69)Y_(0.3)Ob was used in the example I-10, andLa_(0.01)Ce_(0.69)Zr_(0.2)Y_(0.1)Ob was used in the example I-11.

Comparative Example I-1

[0079]FIG. 4 shows the structure of the HC-trap catalyst of thecomparative example I-1. The HC-trap catalyst of the comparative exampleI-1 includes a catalyst layer having the same composition on theupstream and the downstream. As shown in FIG. 4, the catalyst layer 131is formed on the entire region of the HC adsorbent layer 120 prepared bythe similar method to that of the example I-1.

[0080] Similarly to the upper catalyst layer 312 of the example I-2, thecatalyst layer 131 contains Pd-supported Ce-[A]-Ob and Pd-supportedCe—Al₂O₃. As Ce-[A]-Ob, La_(0.01)Ce_(0.69)Zr_(0.3)Ob was used.

[0081] The catalyst layer 131 was prepared under the followingconditions.

[0082] <Catalyst Layer 131>

[0083] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a palladium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pd supported alumina powder (powder-a) was obtained. ThePd concentration of this “powder-a” was 4.0%.

[0084] Cerium oxide powder (Ce 69 mol %) containing 1 mol % of La and 30mol % of Zr was impregnated with a palladium nitrate aqueous solution,or sprayed therewith while the cerium oxide powder was stirred at a highspeed. After the cerium oxide powder was dried at 150° C. for 24 hours,the dried cerium oxide powder was baked at 400° C. for an hour, and thenat 600° C. for an hour, and thus Pd supported cerium oxide powder(powder-h) was obtained. The Pd concentration of this “powder-h” was2.0%.

[0085] 565 g of the foregoing Pd supported alumina powder (powder-a),283 g of the foregoing Pd supported cerium oxide powder (powder-h), 120g of nitric acid alumina sol (12 g, in Al₂O₃, of sol obtained by adding10% of nitric acid to 10% of boehmite alumina), 40 g of barium carbonate(27 g of BaO) and 2000 g of pure water were poured into a magnetic ballmill, then were mixed and milled, and thus a slurry solution wasobtained. This slurry solution was coated on the HC adsorbent layer 120,dried after removing extra slurry in the cells by an air flow, and bakedat 400° C. for an hour. The weight of the coating layer after the bakingwas 90.0 g/dm³. Thus, the catalyst layer 131 was obtained.

[0086] The noble metal supported amount on the entire catalyst layerswas 2.83 g/dm³ for Pd.

Comparative Example I-2

[0087] The upper catalyst layer formed by the similar method to that ofthe catalyst layer 131 of the comparative example I-1 was formed on theregion of about 90% of the overall length of the cell from the upperend. Note that 637 g of the Pd supported cerium oxide powder(concentration of supported Pd 4%) and 708 g of the Pd supported aluminapowder (concentration of supported Pd 4%) were used. The lower catalystlayer was formed on the region of about 10% of the overall length of thecell from the lower end.

Comparative Example I-3

[0088] The Pd-supported Ce-[A]-Ob was not contained in the uppercatalyst layer. The preparation method of the upper catalyst layer wasconformed to the preparation method of the upper catalyst layer of theexample I-2, and the Pd supported cerium oxide powder was not used, but708 g of the Pd supported alumina powder (concentration of supported Pd4%) was used. For the lower catalyst layer, the same one as the lowercatalyst layer I-1 was used.

Comparative Example I-4

[0089] The catalyst of the example I-1 was cut into a catalyst unitwhere the upper catalyst layer was formed and a catalyst unit where thelower catalyst layer was formed, and these two catalysts were arrayed intandem. Other than this, the catalyst was prepared under the similarcondition to that of the example I-1.

Comparative Example I-5

[0090] The arrangement of the upper catalyst layer and the lowercatalyst layer of the catalyst of the example I-1 was inverted. Otherthan this, the catalyst was prepared under the similar condition to thatof the example I-1.

[0091] <Method of Evaluation>

[0092] Each of the catalysts of the examples I-1 to I-11 and thecomparative examples I-1 to I-5 was used as the HC-trap catalyst 100 ofthe exhaust gas purifying system shown in FIG. 5, and the HC adsorptionrate and the HC purification efficiency were measured. In this purifyingsystem, the three-way catalyst 200 (capacity 1.0 dm³ (L)) was disposedin the upstream region of the exhaust gas passage 400 for the gasexhausted from the engine 300, and the HC-trap catalyst 100 (capacity1.0 dm³ (L)) of the example or the comparative example was disposed inthe downstream thereof. Note that an air/fuel ratio sensor 500 and anoxygen sensor 600 were provided on the exhaust gas passage 400. Thethree-way catalyst 200 was prepared under the following conditions.Moreover, for the evaluation conditions, the following were used. Theresults were shown in the table of FIG. 6C.

[0093] It was found out that the HC-trap catalysts of the examples I-1to I-11 were superior in HC adsorbing/purifying capability to thecatalysts of the comparative examples I-1 to I-5.

[0094] <Three-Way Catalyst>

[0095] 530 g of the “powder-a”, 236 g of the “powder-b”, 70 g of nitricacid alumina sol (14 g, in Al₂O₃, of sol obtained by adding 10% ofnitric acid to 10% of boehmite alumina), 40 g of barium carbonate (27 gof BaO) and 1000 g of pure water were poured into a magnetic ball mill,then were mixed and milled, and thus a slurry solution was obtained.This slurry solution was coated on a monolithic carrier of 900 cells/2mills (139.5 cells/cm², wall thickness 0.0051 cm) and of a catalystcapacity 1.0 dm³, dried after removing extra slurry in the cells by anair flow, and baked at 400° C. for an hour. The coating was carried outsuch that the weight of the coating layer after the baking was 78 g/dm³.Thus, a “catalyst-a” was obtained.

[0096] 313 g of the “powder-e”, 100 g of zirconium oxide powdercontaining 1 mol % of La and 20 mol % of Ce, 170 g of nitric acidalumina sol (17 g, in Al₂O₃, of sol obtained by adding 10% of nitricacid to 10% of boehmite alumina) and 1000 g of pure water were pouredinto a magnetic ball mill, then were mixed and milled, and thus a slurrysolution was obtained. This slurry solution was coated on the“catalyst-a”, dried after removing extra slurry in the cells by an airflow, and baked at 400° C. for an hour. The coating was carried out suchthat the weight of the coating layer after the baking was 43 g/dm³, andthus a catalyst was obtained. The noble metal supported amount on thecatalyst was 2.35 g/dm³ for Pd, and 0.47 g/dm³ for Rh. <Durabilitycondition> Engine displacement 3000 cc Fuel gasoline (Nisseki Dash)Catalyst inlet gas temperature 650° C. Time of durability 100 hours<Vehicle performance test> Engine displacement In-line four-cylinder 2.0L engine by Nissan Motor Co., Ltd. Method of evaluation A-bag of LA4-CHof North America exhaust gas testing method

[0097] (Second Embodiment)

[0098] Similarly to the exhaust gas purifying catalyst according to thefirst embodiment, the exhaust gas purifying catalyst according to thesecond embodiment of the present invention is a catalyst in which thecatalyst composition and the catalyst structure are changed in theupstream region and the downstream region.

[0099] Similarly to the HC-trap catalyst according to the firstembodiment, the HC-trap catalyst according to the second embodiment ischaracterized in that the upper catalyst layer contains more O2-storagematerial than the lower catalyst layer, and that the lower catalystlayer contains a catalyst having a wider activation range than the lowercatalyst layer. Furthermore, the HC-trap catalyst according to thesecond embodiment is characterized in that the cross-sectional area ofthe exhaust gas passage in the downstream region thereof is narrow.Since the exhaust gas passage in the downstream region is narrow, thecontact of the exhaust gas and the lower catalyst layer is enhanced, andthe lower catalyst layer is heated rapidly by the heat of the exhaustgas and can be activated. Therefore, the HC purification efficiency ofthe HC-trap catalyst can be improved.

[0100] The HC-trap catalyst according to the second embodiment will bedescribed below with reference to the drawings.

[0101]FIGS. 7A to 7C show the structural example of the cell of thefirst HC-trap catalyst according to the second embodiment. As shown inFIGS. 7A to 7C, the HC adsorbent layer 20 mainly containing zeolite isformed on the carrier 10 of each cell. The film thickness of the HCadsorbent layer 20 is made thicker in the downstream region than in theupstream region, and thus the substantial cross-sectional area of theexhaust gas passage in the downstream region becomes smaller than thatin the upstream region. Specifically, in the first HC-trap catalyst, thecoating amount of slurry of the HC adsorbent layer 20 is increased inthe downstream region when the HC adsorbent layer 20 is formed on thecarrier 10, and thus the thickness of the HC adsorbent layer 20 isthickened in the downstream region.

[0102] The upper catalyst layer 331 is stacked in the upstream region ofthe HC adsorbent layer 20, and the lower catalyst layer 341 is stackedin the downstream region of the HC adsorbent layer 20. The uppercatalyst layer 331 contains more O₂-storage material than the lowercatalyst layer 341, and the lower catalyst layer 341 contains thecatalyst having the wider activation range than the upper catalyst layer331.

[0103] Similarly to the HC-trap catalyst according to the firstembodiment, preferably, the upper catalyst layer 331 mainly uses ceriumoxide as an O₂-storage material as the catalyst supporter, and the lowercatalyst layer 341 mainly uses alumina capable of highly dispersing thecatalyst as the catalyst supporter. Preferably, in addition to Pd, thelower catalyst layer 341 mainly uses Pt, Rh or the like having a wideactivation range.

[0104] Concretely, the upper catalyst layer 331 uses Pd-supportedCe-[A]-Ob as a main component. The reference code A denotes at least oneelement selected from the group consisting of zirconium, lanthanum,yttria, praseodymium and neodymium. Note that Pd-supported Ce—Al₂O₃ maybe mixed in the upper catalyst layer 331. The lower catalyst layer 341may contain any of Pd-supported Ce—Al₂O₃, Pt-supported Ce—Al₂O₃,Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂, or an arbitrarycombination thereof.

[0105] Since the first HC-trap catalyst shown in FIGS. 7A to 7C use theabove-described upper catalyst layer 331 and lower catalyst layer 341,the HC purification efficiency thereof can be improved by the similareffect to that of the HC-trap catalyst according to the firstembodiment. Moreover, since the cross-sectional area of the exhaust gaspassage is narrowed in the downstream region, the contact efficiency ofthe lower catalyst layer 341 and the exhaust gas is enhanced, the heatof the exhaust gas heats the catalyst efficiently, and the catalyst canbe activated rapidly. Note that it is desirable to heat up the lowercatalyst layer 341 to a range from 100 to 400° C., and the lowercatalyst layer 341 can also be controlled by use of a temperature sensoror the like during operation.

[0106] Next, FIG. 8 shows the cell structure of the second HC-trapcatalyst according to the second embodiment.

[0107] As shown in FIG. 8, in the second HC-trap catalyst, the HCadsorbent layer 20 mainly containing zeolite is formed only in theupstream region on the carrier 10 of each cell, and a heat-resistantinorganic oxide layer 22 mainly containing alumina is formed in thedownstream region of the carrier 10. In the second HC-trap catalyst, thefilm thickness of the heat-resistant inorganic oxide layer 22 is madethicker than that of the HC adsorbent layer 20 in the upstream region,and thus the cross-sectional area of the exhaust gas passage is narrowedin the downstream region.

[0108] The upper catalyst layer 331 is formed on the HC adsorbent layer20, and the lower catalyst layer 341 is formed on the heat-resistantinorganic oxide layer 22. For the upper catalyst layer 331 and the lowercatalyst layer 341, a similar composition to that of the first HC-trapcatalyst shown in FIGS. 7A to 7C can be used.

[0109] Also in the second HC-trap catalyst, the substantialcross-sectional area of the exhaust gas passage in the downstream regionis narrowed by the heat-resistant inorganic oxide layer 22 formed in thedownstream region of the cell, similarly to the first HC-trap catalyst.Therefore, the contact efficiency of the lower catalyst layer 341 andthe exhaust gas is enhanced, thus making it possible to heat up thelower catalyst layer 341 rapidly.

[0110] The heat-resistant inorganic oxide layer 22 formed in thedownstream region is denser than the HC adsorbent layer 20 having a highporosity, which is made of zeolite and the like. Therefore, when the HCdiffused in the HC adsorbent layer 20 reach the heat-resistant inorganicoxide layer 22 in the downstream region, a diffusion flow thereof isdisturbed there. Moreover, the diffusion rate of the HC slows down inthe heat-resistant inorganic oxide layer 22. Since the desorption of theHC can be delayed in such a manner, the purification efficiency for thedesorbed HC by the lower catalyst layer 341 can be improved. Note thatthe heat-resistant inorganic oxide layer 22 preferably containsγ-alumina having a particle diameter ranging from 1 to 3 μm as a maincomponent.

[0111] The upper catalyst layer 331 formed on the HC adsorbent layer 20can use Pd-supported Ce-[A]-Ob as a main component. Pd-supportedCe—Al₂O₃ may be mixed in the upper catalyst layer 331. The lowercatalyst layer 341 formed on the heat-resistant inorganic oxide layer 22may contain any of Pd-supported Ce—Al₂O₃, Pt-supported Ce—Al₂O₃,Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂, or an arbitrarycombination thereof. In the HC-trap catalysts shown in FIGS. 9 to 11,the compositions of the upper catalyst layer and the lower catalystlayer of the HC-trap catalyst shown in FIG. 8 are changed.

[0112] Next, FIG. 12 shows the cell structure of the third HC-trapcatalyst.

[0113] As shown in FIG. 12, the third HC-trap catalyst is an HC-trapcatalyst that uses a first honeycomb carrier 11 having a small number ofcells in the upstream region and uses a second honeycomb carrier 12having a larger number of cells than the first honeycomb carrier 11 inthe downstream region. The first honeycomb carrier 11 and the secondhoneycomb carrier 12 may be made monolithic completely. Alternatively,separate carriers may be used so as to be adjacent to each other.

[0114] The HC adsorbent layer 20 is formed on the first honeycombcarrier 11 provided on the upstream side, and the upper catalyst layer331 is formed on the HC adsorbent layer 20. The lower catalyst layer 341is formed directly on the second honeycomb carrier 12 provided on thedownstream side. Preferably, the compositions of the respective catalystlayers 331 and 341 are made similar to that of the first HC-trapcatalyst.

[0115] As described above, since the third HC-trap catalyst uses thesecond honeycomb carrier having a larger number of cells in thedownstream region, the cross section of the exhaust gas passage in eachcell of the downstream region can be narrowed. Therefore, the contactefficiency of the exhaust gas and the lower catalyst layer 341 formed onthe second honeycomb carrier 12 is enhanced, and the temperatureincrease of the lower catalyst layer 341 can be accelerated. Concretely,the number of cells of the second honeycomb carrier 12 is desirablytwice to five times the number of cells of the first honeycomb carrier11.

[0116] Although the lower catalyst layer 341 is formed directly on thesecond honeycomb carrier 12, an HC adsorbent layer may be interposedbetween the second honeycomb carrier 12 and the lower catalyst layer341.

[0117] The upper catalyst layer 331 can use Pd-supported Ce-[A]-Ob as amain component. Moreover, Ce—Al₂O₃ may be mixed in the upper catalystlayer 331. The lower catalyst layer 341 formed on the heat-resistantinorganic oxide layer 22 contain any of Pd-supported Ce—Al₂O₃,Pt-supported Ce—Al₂O₃, Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂, oran arbitrary combination thereof. In the HC-trap catalysts shown inFIGS. 13 to 15, the compositions of the upper catalyst layer and thelower catalyst layer of the third HC-trap catalyst are changed.

[0118] The first to third HC-trap catalysts according to the secondembodiment have been described above. In the exhaust gas passage of eachof the HC-trap catalysts, the contact rate and time of the exhaust gason the downstream side of the catalyst portion can be increased when theaverage cross-sectional area A₁ of the exhaust gas passage on theupstream side and the average cross-sectional area A₂ of the exhaust gaspassage on the downstream side satisfy a relationship of A₁:A₂=1:0.99 to0.6. The average cross-sectional area Al on the upstream side means thesubstantial average cross-sectional area of the exhaust gas passage inthe region where the upper catalyst layer 331 is formed. The averagecross-sectional area A₂ on the downstream side means the substantialaverage cross-sectional area of the exhaust gas passage in the regionwhere the lower catalyst layer 341 is formed.

[0119] When the honeycomb carrier is of a monolithic type, preferably,the upper catalyst layer 331 is formed on the upstream region of eachcell, which occupies 50 to 90% of the overall length thereof. When thefirst honeycomb carrier 11 and the second honeycomb carrier 12 areseparately formed, desirably, 50 to 90% of the overall length of cells,which is obtained by adding the cell length of the first honeycombcarrier 11 and the cell length of the second honeycomb carrier 12, isset as the cell length of the first honeycomb carrier 11. Specifically,the portion occupying 50 to 90% of the overall length of the cell in theupstream region is desirably set as the upper catalyst layer 331.

[0120] For the materials of the HC adsorbent layer 20 and the honeycombcarriers 10 to 12 used in the HC-trap catalyst according to the secondembodiment, similar ones to those of the HC-trap catalyst according tothe first embodiment can be used.

EXAMPLES II

[0121] The tables of FIGS. 22A and 22B show specifications of catalystsin examples II-1 to II-11, and the table of FIG. 22C showsspecifications of catalysts in comparative examples II-1 to II-6.

Example II-1

[0122]FIGS. 7A to 7C show the structure of the HC-trap catalyst of theexample II-1. In the HC-trap catalyst of the example II-1, the HCadsorbent layer 20 in the downstream region was thickened, and thus thecross-sectional area of the exhaust gas passage in the downstream regionwas narrowed. Moreover, the upper catalyst layer 331 mainly containsPd-supported Ce-[A]-Ob. Concretely, La_(0.01)Ce_(0.69)Zr_(0.3)Ob wasused as Ce-[A]-Ob. The lower catalyst layer 341 mainly containsPd-supported Ce—Al₂O₃.

[0123] The respective layers were prepared by the following methods.

[0124] <HC Adsorbent Layer 20>

[0125] 800 g of β-zeolite powder (Si/2Al=35), 1333.3 g of silica sol(solid part 15 wt %) and 1000 g of pure water were poured into a ballmill pot made of alumina, then were milled for 60 minutes, and thus aslurry solution was obtained. This slurry solution was coated on amonolithic carrier of 300 cells/6 mills (46.5 cells/cm², wall thickness0.0152 cm) and of a catalyst capacity 1.0 dm³(L), dried for 30 minutesin an air flow of 50° C. after removing extra slurry in the cells by anair flow, then baked at 400° C. for an hour after drying for 15 minutesin an air flow of 150° C. The coating step was repeated until the amountof coating reached 300 g/dm³ after the baking. Furthermore, theforegoing slurry was coated on the exhaust gas passage region of aboutone-fourth of the overall length of the cell of the monolithic carrierfrom the lower end until the amount of coating reached 50 g/dm³ afterthe baking. Thus, the HC adsorbent layer 20 was formed.

<Upper Catalyst Layer 331>

[0126] Cerium oxide powder (Ce 67 mol %) containing 1 mol % of La and 32mol % of Zr was impregnated with a palladium nitrate aqueous solution,or sprayed therewith while the cerium oxide powder was stirred at a highspeed. After the cerium oxide powder was dried at 150° C. for 24 hours,the dried cerium oxide powder was baked at 400° C. for an hour, and thenat 600° C. for an hour, and thus Pd-supported cerium oxide powder(powder-b) was obtained. The Pd concentration of this “powder-b” was1.0%.

[0127] 628 g of the foregoing Pd-supported cerium oxide powder(powder-b), 390 g of nitric acid alumina sol (39 g, in Al₂O₃, of solobtained by adding 10% of nitric acid to 10% of boehmite alumina) and2000 g of pure water were poured into a magnetic ball mill, then weremixed and milled, and thus a slurry solution was obtained. This slurrysolution was coated on three-fourth portion of the foregoing HCadsorbent layer 20, which corresponds to the upstream side, dried afterremoving extra slurry in the cells by an air flow, and baked at 400° C.for an hour. The weight of the coating layer after the baking was 66.7g/dm³. Thus, the upper catalyst layer 331 was obtained.

[0128] <Lower Catalyst Layer 341>

[0129] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a palladium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pd supported alumina powder (powder-a) was obtained. ThePd concentration of this “powder-a” was 8.0%.

[0130] 275 g of the foregoing Pd-supported alumina powder (powder-a),240 g of nitric acid alumina sol (24 g, in Al₂O₃, of sol obtained byadding 10% of nitric acid to 10% of boehmite alumina), 50 g of bariumcarbonate (34 g of BaO) and 2000 g of pure water were poured into amagnetic ball mill, then were mixed and milled, and thus a slurrysolution was obtained. This slurry solution was coated on the one-fourthportion in the downstream region of the HC adsorbent layer 20, driedafter removing extra slurry in the cells by an air flow, and baked at400° C. for an hour. The weight of the coating layer after the bakingwas 33.3 g/dm³. Thus, the lower catalyst layer 341 was obtained.

Example II-2

[0131]FIG. 8 shows the structure of the HC-trap catalyst of the exampleII-2. In the HC-trap catalyst of the example II-2, the thickheat-resistant inorganic oxide layer 22 was formed in the downstreamregion, and thus the cross-sectional area of the exhaust gas passage wasnarrowed in the downstream region. Note that y-alumina was used as theheat-resistant inorganic material. The upper catalyst layer 331 mainlycontains Pd-supported Ce-[A]-Ob. Concretely,La_(0.01)Ce_(0.69)Pr_(0.3)Ob was used as Ce-[A]-Ob. The lower catalystlayer 341 mainly contains Pt-supported Ce—Al₂O₃.

[0132] The HC adsorbent layer 20 and the heat-resistant inorganic oxidelayer 22 were prepared by the following methods.

[0133] The upper catalyst layer 331 and the lower catalyst layer 341were prepared under the similar conditions to those of the example II-1.However, instead of Zr, Pr was contained in the cerium oxide powder whenpreparing the upper catalyst layer 331.

[0134] <HC Adsorbent Layer 20>

[0135] 800 g of β-zeolite powder (Si/2Al=35), 1333.3 g of silica sol(solid part 15 wt %) and 1000 g of pure water were poured into a ballmill pot made of alumina, then were milled for 60 minutes, and thus aslurry solution was obtained. This slurry solution was coated on amonolithic carrier of 300 cells/6 mills (46.5 cells/cm², wall thickness0.0152 cm) and of a catalyst capacity 1.0 L, dried for 30 minutes in anair flow of 50° C. after removing extra slurry in the cells by an airflow, then baked at 400° C. for an hour after drying for 15 minutes inan air flow of 50° C. The coating step was repeated on a three-fourthportion located on the upstream side of the exhaust gas until the amountof coating reached 263 g/dm³ after the baking. Thus, the HC adsorbentlayer 20 was obtained.

[0136] <Heat-Resistant Inorganic Oxide Layer 22>

[0137] 950 g of γ-alumina, 500 g of nitric acid alumina sol and 1000 gof pure water were poured into a magnetic ball mill, then were mixed andmilled, and thus a slurry solution was obtained. The average particlediameter in this case was 1.0 to 1.5 μm. This slurry solution was coatedon a one-fourth portion on the exhaust gas downstream side of theforegoing HC adsorbent layer 20, and dried after removing extra slurryin the cells by an air flow, then baked at 400° C. for an hour. Theweight of the coating layer after the baking was 90 g/dm³. Thus, theheat-resistant inorganic oxide layer 22 was obtained.

Example II-3

[0138]FIG. 9 shows the structure of the HC-trap catalyst of the exampleII-3. In the HC-trap catalyst of the example II-3, the thickheat-resistant inorganic oxide layer 22 was formed in the downstreamregion, and thus the cross-sectional area of the exhaust gas passage wasnarrowed in the downstream region. The HC adsorbent layer 20 and theheat-resistant inorganic oxide layer 22 were prepared under the sameconditions as those of the example II-2.

[0139] The upper catalyst layer 332 mainly contains Pd-supportedCe-[A]-Ob and Pd-supported Ce—Al₂O₃. Concretely,La_(0.01)Ce_(0.69)Nd_(0.3)Ob was used as Ce-[A]-Ob. The lower catalystlayer 341 contains Pd-supported Ce—Al₂O₃.

[0140] The lower catalyst layer 341 was prepared under the sameconditions as those of the example II-2. The upper catalyst layer 332was prepared under the following conditions.

[0141] <Upper Catalyst Layer 332>

[0142] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a palladium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pd-supported alumina powder (powder-c) was obtained. ThePd concentration of this “powder-c” was 1.0%.

[0143] Cerium oxide powder (Ce 69 mol %) containing 1 mol % of La and 30mol % of Nd was impregnated with a palladium nitrate aqueous solution,or sprayed therewith while the cerium oxide powder was stirred at a highspeed. After the cerium oxide powder was dried at 150° C. for 24 hours,the dried cerium oxide powder was baked at 400° C. for an hour, and thenat 600° C. for an hour, and thus Pd-supported cerium oxide powder(powder-d) was obtained. The Pd concentration of this “powder-d” was1.0%.

[0144] 400 g of the foregoing Pd-supported alumina powder (powder-c),228 g of the foregoing Pd-supported cerium oxide powder (powde-d), 140 gof nitric acid alumina sol (14 g, in Al₂O₃, of sol obtained by adding10% of nitric acid to 10% of boehmite alumina), 25 g of barium carbonate(17 g of BaO) and 1000 g of pure water were poured into a magnetic ballmill, then were mixed and milled, and thus a slurry solution wasobtained. This slurry solution was coated on the HC adsorbent layer 20formed in advance, dried after removing extra slurry in the cells by anair flow, and baked at 400° C. for an hour. The weight of the coatinglayer after the baking was 66.7 g/dm³. Thus, the upper catalyst layer332 was obtained.

Example II-4

[0145]FIG. 10 shows the structure of the HC-trap catalyst of the exampleII-4. In the HC-trap catalyst of the example II-4, the thickheat-resistant inorganic oxide layer 22 was formed in the downstreamregion, and thus the cross-sectional area of the exhaust gas passage wasnarrowed in the downstream region. The HC adsorbent layer 20 and theheat-resistant inorganic oxide layer 22 were prepared under the sameconditions as those of the example II-2.

[0146] The upper catalyst layer 333 mainly contains Pd-supportedCe-[A]-Ob. Concretely, La_(0.01)Ce_(0.69)Pr_(0.2)Nd_(0.1)Ob was used asCe-[A]-Ob. The lower catalyst layer 343 contains Pt-supported Ce—Al₂O₃,Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂. The upper catalyst layer333 was prepared by a similar method of the example II-1. However,instead of Zr, Pr and Nd were contained in the cerium oxide powder whenpreparing the upper catalyst layer 333. Also the Pd concentration of thePd-supported cerium oxide powder was 3.0%.

[0147] The lower catalyst layer 343 was prepared by the followingpreparation method.

<Lower Catalyst Layer 343>

[0148] Alumina powder (Al 97 mol %) containing 3 mol % of Zr wasimpregnated with a rhodium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Rh-supported alumina powder (powder-e) was obtained. TheRh concentration of this “powder-e” was 1.5%.

[0149] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a dinitro diamine platinum aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pt supported alumina powder (powder-f) was obtained. ThePt concentration of this “powder f” was 1.5%.

[0150] Zirconium oxide powder containing 1 mol % of La and 20 mol % ofCe was impregnated with a dinitro diamine platinum aqueous solution, orsprayed therewith while the zirconium oxide powder was stirred at a highspeed. After the zirconium oxide powder was dried at 150° C. for 24hours, the dried zirconium oxide powder was baked at 400° C. for anhour, and then at 600° C. for an hour, and thus Pt supported zirconiumoxide powder (powder-g) was obtained. The Pt concentration of this“powder-g” was 1.5%.

[0151] 157 g of the foregoing Rh supported alumina powder (powder-e),236 g of the foregoing Pt supported alumina powder (powder-f), 236 g ofthe foregoing Pt supported zirconium oxide powder (powder-g) and 380 gof nitric acid alumina sol were poured into a magnetic ball mill, thenwere mixed and milled, and thus a slurry solution was obtained. Thisslurry solution was coated on the heat-resistant inorganic oxide layer22, dried after removing extra slurry in the cells by an air flow, andbaked at 400° C. for an hour. The weight of the coating layer after thebaking was 33.3 g/dm³. Thus, the lower catalyst layer 343 was obtained.

Example II-5

[0152]FIG. 11 shows the structure of the HC-trap catalyst of the exampleII-5. In the HC-trap catalyst of the example II-5, the thickheat-resistant inorganic oxide layer 22 was formed in the downstreamregion, and thus the cross-sectional area of the exhaust gas passage wasnarrowed in the downstream region. The HC adsorbent layer 20 and theheat-resistant inorganic oxide layer 22 were prepared under the sameconditions as those of the example II-2.

[0153] The upper catalyst layer 334 mainly contains Pd-supportedCe-[A]-Ob and Pd-supported Ce—Al₂O₃. Concretely,La_(0.01)Ce_(0.69)Zr_(0.2)Pr_(0.1)Ob was used as Ce-[A]-Ob. The lowercatalyst layer 343 contains Pt-supported Ce—Al₂O₃, Rh-supportedZr—Al₂O₃, Pt-supported LaCe—ZrO2.

[0154] The upper catalyst layer 334 was prepared by a similar method ofthe example II-3. Note that, 400 g of Pd-supported alumina powder having3.0% of Pd and 228 g of Pd-supported cerium oxide powder having 3% of Pdwere used. The lower catalyst layer 343 was prepared under the sameconditions as those of the example II-4.

Example II-6

[0155]FIG. 12 shows the structure of the HC-trap catalyst of the exampleII-6. In the HC-trap catalyst of the example II-6, the first honeycombcarrier 11 having the number of cells of 300 (46.5 cells/cm²) was usedin the upstream region, and the second honeycomb carrier 12 having thenumber of cells of 900 (139.5 cells/cm²) was used in the downstreamregion, and thus the cross-sectional area of the exhaust gas passage wasnarrowed in the downstream region.

[0156] The HC adsorbent layer 20 was prepared on the first honeycombcarrier 11, and the upper catalyst layer 331 containing Pd-supportedCe-[A]-Ob was prepared on the HC adsorbent layer 20. Concretely,La_(0.01)Ce_(0.69)Pr_(0.3)Ob was used as Ce-[A]-Ob. The lower catalystlayer 341 mainly containing Pd-supported Ce—Al₂O₃ was formed directly onthe second honeycomb carrier 12. In the case of using the HC-trapcatalyst of this example in the exhaust gas purifying system, the firsthoneycomb carrier 11 and the second honeycomb carrier 12 were arrayed inone catalyst converter so as to be adjacent to each other.

[0157] The preparation conditions of the respective layers will bedescribed below.

[0158] <HC Adsorbent Layer 20>

[0159] 800 g of β-zeolite powder (Si/2Al=35), 1333.3 g of silica sol(solid part 15 wt %) and 1000 g of pure water were poured into a ballmill pot made of alumina, then were milled for 60 minutes, and thus aslurry solution was obtained. This slurry solution was coated on thefirst monolithic carrier 11 of 300 cells/6 mills (46.5 cells/cm², wallthickness 0.0152 cm) and of a catalyst capacity 1.0 dm³ (L). Then, theslurry solution was dried for 30 minutes in an air flow of 50° C. afterremoving extra slurry in the cells by an air flow, then baked at 400° C.for an hour after drying for 15 minutes in an air flow of 150° C. Thecoating step was repeated until the amount of coating reached 350 g/dm³after the baking. Thus, the HC adsorbent layer 20 was obtained.

[0160] <Upper Catalyst Layer 331>

[0161] Cerium oxide powder (Ce 67 mol %) containing 1 mol % of La and 32mol % of Zr was impregnated with a palladium nitrate aqueous solution,or sprayed therewith while the cerium oxide powder was stirred at a highspeed. After the cerium oxide powder was dried at 150° C. for 24 hours,the dried cerium oxide powder was baked at 400° C. for an hour, and thenat 600° C. for an hour, and thus Pd-supported cerium oxide powder(powder-b) was obtained. The Pd concentration of this “powder-b” was1.0%.

[0162] 628 g of the foregoing Pd-supported cerium oxide powder(powder-b), 390 g of nitric acid alumina sol (39 g, in Al₂O₃, of solobtained by adding 10% of nitric acid to 10% of boehmite alumina) and2000 g of pure water were poured into a magnetic ball mill, then weremixed and milled, and thus a slurry solution was obtained. This slurrysolution was coated on the HC adsorbent layer 20 formed on the firsthoneycomb carrier 11, dried after removing extra slurry in the cells byan air flow, and baked at 400° C. for an hour. The coating step wasrepeated until the amount of coating reached 66.7 g/dm³ after thebaking. Thus, the upper catalyst layer 331 was obtained.

[0163] <Lower Catalyst Layer 341>

[0164] Alumina powder (Al 97 mol %) containing 3 mol % of Ce wasimpregnated with a palladium nitrate aqueous solution, or sprayedtherewith while the alumina powder was stirred at a high speed. Afterthe alumina powder was dried at 150° C. for 24 hours, the dried aluminapowder was baked at 400° C. for an hour, and then at 600° C. for anhour, and thus Pd-supported alumina powder (powder-a) was obtained. ThePd concentration of this “powder-a” was 8.0%.

[0165] 275 g of the foregoing Pd-supported alumina powder (powder-a),240 g of nitric acid alumina sol (24 g, in Al₂O₃, of sol obtained byadding 10% of nitric acid to 10% of boehmite alumina), 50 g of bariumcarbonate (34 g of BaO) and 2000 g of pure water were poured into amagnetic ball mill, then were mixed and milled, and thus a slurrysolution was obtained. This slurry solution was coated on the secondhoneycomb carrier 12 of 900 cells/2 mills (139.5 cells/cm², wallthickness 0.005 cm) and of a catalyst capacity 0.25 dm³(L), dried afterremoving extra slurry in the cells by an air flow, and baked at 400° C.for an hour. The weight of the coating layer after the baking was 33.3g/dm³. Thus, the lower catalyst layer 341 was obtained.

Example II-7

[0166]FIG. 13 shows the structure of the example II-7. The firsthoneycomb carrier 11 having the number of cells of 300 was used in theupstream region, and the second honeycomb carrier 12 having the numberof cells of 900 was used in the downstream region. The HC-trap catalystlayer was prepared, which contained Pd-supported Ce-[A]-Ob andPd-supported Ce—Al₂O₃ in the upper catalyst layer 332, and containedPd-supported Ce—Al₂O₃ in the lower catalyst layer 341.

[0167] The HC adsorbent layer 20 and the upper catalyst layer 332 wereprepared under the similar conditions to those of the example II-3. Thelower catalyst layer 341 was prepared under the same conditions as thoseof the example II-6. Note that La_(0.01)Ce_(0.69)Nd_(0.3)Ob was used asCe-[A]-Ob.

Example II-8

[0168]FIG. 14 shows the structure of the example II-8. The firsthoneycomb carrier 11 having the number of cells of 300 (46.5 cells/cm²)was used in the upstream region, and the second honeycomb carrier 12having the number of cells of 900 (139.5 cells/cm²) was used in thedownstream region. Pd-supported Ce-[A]-Ob is contained in the uppercatalyst layer 331. Pd-supported Ce—Al₂O₃, Pt-supported Ce—Al₂O₃,Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂ are contained in the lowercatalyst layer 343.

[0169] The HC adsorbent layer 20 and the upper catalyst layer 333 wereprepared under the similar conditions to those of the example II-4. Thelower catalyst layer 343 was prepared under the same conditions as thoseof the example II-4. Note that La_(0.01)Ce_(0.69)Pr_(0.2)Nd_(0.1)Ob wasused as Ce-[A]-Ob.

Example II-9

[0170]FIG. 15 shows the structure of the example II-9. Similarly to theexample II-6, the first honeycomb carrier 11 having the number of cellsof 300 was used in the upstream region, and the second honeycomb carrier12 having the number of cells of 900 was used in the downstream region.Pd-supported Ce-[A]-Ob and Pd-supported Ce—Al₂O₃ are contained in theupper catalyst layer 334. Pd-supported Ce—Al₂O₃, Pt-supported Ce—Al₂O₃,Rh-supported Zr—Al₂O₃, Pt-supported LaCe—ZrO₂ are contained in the lowercatalyst layer 343.

[0171] The HC adsorbent layer 20 and the upper catalyst layer 332 wereprepared under the similar conditions as those of the example II-7. Thelower catalyst layer 343 was prepared under the same conditions as thoseof the example II-8. Note that La_(0.01)Ce_(0.69)Zr_(0.2)Pr_(0.1)Ob wasused as Ce-[A]-Ob.

Example II-10

[0172] The HC-trap catalyst of the example II-10 has a same structure ofthe example II-1 shown in FIGS. 7A to 7C. The HC adsorbent layer in thedownstream region was thickened, and thus the cross-sectional area ofthe exhaust gas passage in the downstream region was narrowed. TheHC-trap catalyst of the example II-10 was prepared using a similarmethod of the example II-1. Note that La_(0.01)Ce_(0.69)Y_(0.3)Ob wasused as Ce-[A]-Ob in the upper catalyst layer.

Example II-11

[0173] The HC-trap catalyst of the example II-11 has a same structure ofthe example II-7 shown in FIG. 13. The HC-trap catalyst of the exampleII-11 was prepared using a similar method of the example II-7. Note thatLa_(0.01)Ce_(0.69)Zr_(0.2)Y_(0.1)Ob was used as Ce-[A]-Ob in the uppercatalyst layer.

Comparative Example II-1

[0174]FIG. 16 shows the structure of the comparative example II-1. Thecomparative example II-1 has the structure in which the upstream regionand the downstream region of the structure of the example II-1 areinverted. Specifically, the HC adsorbent layer 120 of the upstreamregion was thickened, and thus the cross-sectional area of the upstreampassage was narrowed. Moreover, the upper catalyst layer 1341 wascomposed similarly to the lower catalyst layer 341 of the example II-1,and the lower catalyst layer 1331 was composed similarly to the uppercatalyst layer 331 of the example II-2. Fore other preparationconditions, similar ones to those of the example II-1 were used.

Comparative Example II-2

[0175]FIG. 17 shows the structure of the comparative example II-2. Thecomparative example II-2 is constituted such that the thickness of theHC adsorbent layer 120 is made even from the upstream to the downstreamin the structure of the example II-1. For other conditions, similar onesto those of the example II-1 were used.

Comparative Example II-3

[0176]FIG. 18 shows the structure of the comparative example II-3. Thecomparative example II-3 has the structure in which the upstream regionand the downstream region of the structure of the example II-2 areinverted. Specifically, the thick heat-resistant inorganic oxide layer122 was formed in the upstream region, and thus the cross-sectional areaof the upstream passage was narrowed. Moreover, the upper catalyst layer1351 was composed similarly to the lower catalyst layer 341 of theexample II-2, and the lower catalyst layer 1331 was composed similarlyto the upper catalyst layer 331 of the example II-2. Fore otherpreparation conditions, similar ones to those of the example II-2 wereused.

Comparative Example II-4

[0177]FIG. 19 shows the structure of the comparative example II-4. Inthe structure of the example II-2, no heat-resistant inorganic oxidelayer was provided in the downstream region, and the lower catalystlayer 1341 was formed directly on the honeycomb carrier 110. Other thanthis, the similar conditions to those of the example II-2 were used forpreparation.

[0178] Comparative Example II-5

[0179]FIG. 20 shows the structure of the comparative example II-5. Thecomparative example II-5 has the structure in which the upstream regionand the downstream region of the example II-6 are inverted.Specifically, the second honeycomb carrier 112 having a large number ofcells was located in the upstream region, and the first honeycombcarrier 111 having a small number of cells was located in the downstreamregion. For other conditions, the same ones as those of the example II-6were used.

Comparative Example II-6

[0180]FIG. 21 shows the structure of the comparative example II-6. Thecomparative example II-6 was constituted such that the second honeycombcarrier constructed as in the example II-6 was replaced with the carrier130 having the same number of cells as that of the first honeycombcarrier 110, that is, the carrier 130 of 300 cells/6 mills (46.5cells/cm², wall thickness 0.0152 cm).

[0181] <Method of Evaluation>

[0182] Each of the catalysts of the examples II-1 to II-11 and thecomparative examples II-1 to II-6 was used as the HC-trap catalyst 100of the exhaust gas purifying system shown in FIG. 5, and the HCadsorption rate and the HC purification efficiency were measured. Thesame evaluation conditions as in the first embodiment were used. Thesame three-way catalyst as the three-way catalyst 200 in the firstembodiment was used. The results were shown in the table of FIG. 22D.

[0183] It was found out that the HC-trap catalysts obtained in theexamples II-1 to II-11 were superior in HC adsorbing/purifyingcapability to the catalysts obtained in the comparative examples II-1 toII-6.

[0184] The entire contents of Japanese Patent Applications P2001-336188(filed on Nov. 1, 2001), P2001-336227 (filed on Nov. 1, 2001) andP2002-294435 (filed on Oct. 8, 2002) are incorporated herein byreference. Although the inventions have been described above byreference to certain embodiments of the inventions, the inventions arenot limited to the embodiments described above. Modifications andvariations of the embodiments described above will occur to thoseskilled in the art, in light of the above teachings. The scope of theinventions is defined with reference to the following claims.

What is claimed is:
 1. An exhaust gas purifying catalyst, comprising: acarrier having a plurality of cells as exhaust gas passages; an HCadsorbent layer formed on the carrier of each of the cells; and an uppercatalyst layer disposed on an upstream side of each of the exhaust gaspassages on the HC adsorbent layer and a lower catalyst layer disposedon a downstream side of each of the exhaust gas passages on the HCadsorbent layer, wherein the upper catalyst layer includes an O₂-storagematerial more than the lower catalyst layer, and the lower catalystlayer includes a catalyst having a wider activation range than the uppercatalyst layer.
 2. The exhaust gas purifying catalyst according to claim1, wherein the upper catalyst layer includes Pd-supported Ce-[A]-Ob, theCe-[A]-Ob is an oxide compound of cerium and A, and the A is at leastone element selected from the group consisting of zirconium, lanthanum,yttria, praseodymium and neodymium.
 3. The exhaust gas purifyingcatalyst according to claim 2, wherein the upper catalyst layer furtherincludes Pd-supported Ce—Al₂O₃.
 4. The exhaust gas purifying catalystaccording to claim 2, wherein the lower catalyst layer includes, as amain component, at least one selected from the group consisting ofCe—Al₂O₃, Pt-supported Ce—Al₂O₃, Rh-supported Ce—Al₂O₃ and Zr-containedCeO₂.
 5. The exhaust gas purifying catalyst according to claim 1,wherein the carrier is of a monolithic structure type, and the uppercatalyst layer is provided in a range from 50 to 90% of an overalllength of the cell in an upstream region.
 6. An exhaust gas purifyingcatalyst, comprising: a carrier having a plurality of cells as exhaustgas passages; an HC adsorbent layer formed at least on an upstreamregion on the carrier of each of the cells; and a purifying catalystlayer formed on the HC adsorbent layer, wherein the purifying catalystlayer includes an upper catalyst layer disposed on an upstream side ofeach of the exhaust gas passages and a lower catalyst layer disposed ona downstream side of each of the exhaust gas passages, the uppercatalyst layer includes an O₂-storage material more than the lowercatalyst layer, the lower catalyst layer includes a catalyst having awider activation range than the upper catalyst layer, and across-sectional area of the exhaust gas passage is narrower on thedownstream side than on the upstream side.
 7. The exhaust gas purifyingcatalyst according to claim 6, wherein an average passagecross-sectional area A₁ of a region where the upper catalyst layer isformed and an average passage cross-sectional area A₂ of a region wherethe lower catalyst layer is formed satisfy a relationship of:A₁:A₂=1:0.99 to 0.6.
 8. The exhaust gas purifying catalyst according toclaim 6, wherein the HC adsorbent layer covers an entire region of thecarrier of each of the cells, and the HC adsorbent layer formed in adownstream region of the exhaust gas passage is thicker than the HCadsorbent layer formed in the upstream region thereof.
 9. The exhaustgas purifying catalyst according to claim 6, further comprising: aheat-resistant inorganic oxide layer formed in the downstream region onthe carrier of each of the cells, wherein the HC adsorbent layer isformed in the upstream region on the carrier, and the heat-resistantinorganic oxide layer is thicker than the HC adsorbent layer.
 10. Theexhaust gas purifying catalyst according to claim 9, wherein theheat-resistant inorganic oxide layer includes γ-alumina having aparticle diameter of 1 to 3 μm as a main component.
 11. The exhaust gaspurifying catalyst according to claim 9, wherein the upper catalystlayer includes Ce-[A]-Ob as a main component, the Ce-[A]-Ob is an oxidecompound of cerium and A, and the A is at least one element selectedfrom the group consisting of zirconium, lanthanum, yttria, praseodymiumand neodymium.
 12. The exhaust gas purifying catalyst according to claim11, wherein the upper catalyst layer further includes Pd-supportedCe—Al₂O_(3.)
 13. The exhaust gas purifying catalyst according to claim11, wherein the lower catalyst layer includes, as a main component, atleast one selected from the group consisting of Pd-supported Ce—Al₂O₃,Pt-supported Ce—Al₂O₃, Rh-supported Ce—Al₂O₃ and Zr-contained CeO₂. 14.The exhaust gas purifying catalyst according to claim 9, wherein thecarrier is of a monolithic structure type, and the upper catalyst layeris provided in a range from 50 to 90% of an overall length of the cellin the upstream region.
 15. The exhaust gas purifying catalyst accordingto claim 6, wherein the carrier includes: a first carrier disposed onthe upstream side; and a second carrier disposed on the downstream side,the second carrier having a larger number of cells than the firstcarrier.
 16. The exhaust gas purifying catalyst according to claim 15,wherein the number of cells of the second carrier is twice to five timesthe number of cells of the first carrier.
 17. The exhaust gas purifyingcatalyst according to claim 15, wherein in each cell of the firstcarrier, an HC adsorbent layer is formed on the first carrier and theupper catalyst layer is formed on the HC adsorbent layer, and in eachcell of the second carrier, the lower catalyst layer is formed on thesecond carrier.
 18. The exhaust gas purifying catalyst according toclaim 17, wherein the upper catalyst layer includes Ce-[A]-Ob as a maincomponent, the Ce-[A]-Ob is an oxide compound of cerium and A, and the Ais at least one element selected from the group consisting of zirconium,lanthanum, yttria, praseodymium and neodymium.
 19. The exhaust gaspurifying catalyst according to claim 18, wherein the upper catalystlayer further includes Pd-supported Ce—Al₂O₃.
 20. The exhaust gaspurifying catalyst according to claim 19, wherein the lower catalystlayer includes, as a main component, at least one selected from thegroup consisting of Pd-supported Ce—Al₂O₃, Pt-supported Ce—Al₂O₃,Rh-supported Ce—Al₂O₃ and Zr-contained CeO₂.